The invention relates to a hydrostatic adjusting device of a hydraulic machine, the swept volume of which can be adjusted, in accordance with the precharacterizing clause of claim 1, in particular to hydraulic machines which can have their delivery volume or displacement set via a servo adjusting unit and via an adjusting element which is connected mechanically to the latter, for example a swashplate or a bent axis. Here, the concept of the invention comprises all types of hydraulic machines with a servo adjusting means which can be controlled by way of a control slide. The invention preferably relates to hydrostatic adjusting devices for hydraulic machines with a mechanical position return means of the control slide or control piston into the zero position after the adjustment of the hydraulic machine has taken place.
For example, in hydrostatic propulsion drives with a closed or open hydraulic fluid circuit, hydraulic pumps and hydraulic motors are used which usually have controlled servo adjusting means for setting the power output or power consumption of the hydraulic machine. Here, in many cases, the position or swiveling-out of the adjusting element, for example the swashplate or the bent axis, is controlled via a servo adjusting device and the delivery volume of the hydraulic machine is set in this way. This preferably takes place proportionally to an input signal to an actuator, which input signal can be a mechanical force signal, for example as a consequence of a lever movement, or else an electrically generated force signal, for example as a consequence of a current to a proportional magnet. Via a position return device which can be, for example, a mechanical position return means according to DE 10 2004 033 376 B3, the control piston which is deflected by the actuator is moved back into its starting position when the preselected delivery quantity/displacement of the hydraulic machine is reached, which starting position is preferably the neutral position. At the same time, a calibration between the input signal and the real position of the adjusting element is realized in this way, and a regulating circuit between the control apparatus and the adjusting element of the servo adjusting means is closed. In the starting position of the control piston, a return force of the position return apparatus is in force equilibrium with the actuator force.
A hydrostatic adjusting device which forms the generic type for the, for example, infinitely variable adjustment of the delivery flow of a hydraulic delivery device having two delivery directions is described, for example, in DE 10 2008 052 338 B3 and forms the precharacterizing clause of claim 1. The known adjusting device for centering control pistons has a control unit which exhibits a control cylinder which is provided with two inlets for pressurized hydraulic fluid, with two servo connectors for connecting lines to a servo adjusting unit, and at least one outlet to a hydraulic fluid collecting region. A control piston is arranged in the control cylinder, which control piston can be displaced by means of in each case one electromagnet which functions as an actuator and acts on its end faces. By way of an interaction of a plurality of control edges of the control piston with control edges which are configured in the control cylinder, the two inlets or the outlet to the tank can be connected hydraulically alternately to the connecting lines to the servo adjusting unit, as a result of which the servo pressure in the servo cylinders of the servo adjusting unit can be adapted to the performance requirements of the hydraulic machine. Via an adjusting element which is coupled mechanically to the servo pistons, the delivery volume of the hydraulic machine is set. Via a mechanical position return means, the control slide or control piston is returned into its starting position, preferably into the neutral position, by way of elastic prestressing forces after the preselected delivery output has been reached. At the same time, the servo pressure which prevails in the connecting lines to the servo adjusting unit is returned hydraulically at a reduced level via in each case one of two bypass lines to one of the two end sides of the control piston. Here, the pressure level which prevails in the servo chambers of the servo cylinder which are sealed per se is fed at a reduced level to the end faces of the control piston and is discharged further to a hydraulic fluid collecting region of the hydraulic machine, which hydraulic fluid collecting region is under discharge pressure. On account of the return of the servo pressure at a reduced level to the end sides of the control piston, the control piston is held or centred in its zero position in addition to the spring force of the mechanical position return means. The reduction of the pressure in the bypass lines takes place in a conventional way via orifice plates, throttles or nozzles.
The configuration of the hydrostatic adjusting device according to DE 10 2008 052 338 B3 serves exclusively to centre the control piston in the zero position, in the case of a preferably negative overlap of the control edges in the control cylinder. The hydrostatic adjusting device described in the patent DE 10 2008 052 338 B3 for centering control pistons exhibits a certain tendency towards load dependence despite a mechanical position return means which assists load independence of the hydraulic machine.
In the context of the present description of the invention, “load dependence” of an adjusting device is to be understood to mean that the level of the variable external loads which act in each case on the hydraulic machine has an influence via the adjusting device on the level of the pressures which prevail in the servo chambers, and the delivery volume of the hydraulic machine changes in a load-dependent manner as a result of a change in the relative position of the servo piston in the servo cylinder. If, for example, a work machine which is equipped with a hydrostatic drive changes from constant level driving to mountainous driving, the high pressure in the working circuit of the hydrostatic drive rises. The hydraulic pump of the hydrostatic drive tends to endeavour to avoid this pressure increase, by attempting to reduce its delivery quantity, the adjusting element of the servo adjusting means generating an increased force in the direction of the zero position of the pump. This force is supported on the servo piston which is loaded with hydraulic fluid under control pressure, and the servo piston tends to be displaced in the direction of a reduction in size of the servo chamber. If a movement of this type of the servo piston is not permitted as a result of the system, because, for example, a position return device counteracts a load-induced tendency to change the delivery volume of the hydraulic machine, this is called a load-rigid hydrostatic drive. If, in contrast, a load-induced movement of this type of the servo piston is permitted (in a controlled manner), this is called a load-flexible hydrostatic drive, since the load-induced tendency to change the delivery volume of the hydraulic machine is then followed at least partially.
The additional pressure forces on the control piston on account of the returned servo pressures according to DE 10 2008 052 338 B3 act counter to the forces which are exerted on the control piston by the electromagnets, as a result of which the position of the control piston in the control cylinder can be changed. As a further consequence, the pressures in the servo chambers of the servo cylinder and therefore the position of the servo pistons in the respective servo cylinders are adapted in a load-dependent manner. The associated positional change of the adjusting element brings about a load-dependent adaptation of the delivery volume of the hydraulic machine. For example, as the load on the hydrostatic drive rises, the delivery quantity of a hydraulic pump is reduced if the pressure in the working circuit of the hydraulic machine increases. The adjusting unit which is described in DE 10 2008 052 338 B3 is therefore “load-sensing” to a certain extent.
Whether a load-rigid or a load-flexible hydrostatic drive of a work machine is to be used depends, for example, on performance requirements during the use of the hydrostatic drive, the load situation and/or the performance reserves of the drive motor which drives the hydrostatic drive. In many applications, predominantly in the case of mobile work machines, a change between both types of servo adjustment in adaptation to the respective application would be advantageous, that is to say between load-rigid and load-flexible. For example, a load-rigid response of the hydraulic machine would be advantageous for driving at a constant speed, independently of, for example, uphill or downhill slope, and therefore independently of the external load. In another application of the same vehicle, the available drive power is always to be converted, for example, into the maximum possible driving speed depending on the uphill slope or the underlying surface, without overloading the drive, that is to say the drive motor. This load-flexible response is often called automotive driving. A road sweeper machine is to be mentioned as one example which does not restrict the concept of the invention, which road sweeper machine preferably drives at a constant (slow) speed during sweeping, in order to achieve a sweeping result which is as satisfactory as possible. However, the sweeping machine is to be capable of driving at a maximum possible speed during its journey from and to the location of use, for example on the journey to the storage site, in order to keep the time lost as low as possible. It is desirable here that the propulsion drive of the sweeping machine can be operated in a load-rigid manner in the first case and in a load-flexible manner in the second case depending on the use.
The patent DE 10 2004 033 376 B3 describes a load-rigid, hydraulic drive having a servo adjusting device and a mechanical position return means. A load-rigid adjusting device of this type can be controlled, for example, in a load-adapted manner if a microcontroller which is connected to a corresponding sensor system is provided with the corresponding information about the existing load and the available performance. The microcontroller then converts this information into adequate input signals for the proportional magnets of the adjusting device. Costs, reliability, reaction speed and controllability set limits for this embodiment, however.
Conversely, a load-dependent or load-flexible servo adjusting means can also be actuated via a suitable sensor system and a microcontroller in such a way that the load dependence is minimized. A microcontroller, for example, which is connected to a corresponding sensor system also requires the necessary information in this case about the existing load which it converts into adequate input signals for the adjusting unit. In addition to costs, reliability and controllability or control time, as is also the case in the load-rigid system mentioned above, the control operation which is based on electric signals is a technical problem with regard to rapid and precise controllability which can be managed only by the use of complicated and expensive electronics.
The invention is therefore based on the object of providing a hydrostatic adjusting device of the type mentioned at the outset which is suitable for being able to change, during operation of a hydrostatic drive, between a load-rigid and a load-flexible response of the drive and vice versa in accordance with a machine operator and/or the use conditions of the hydraulic drive. The technical solution for an adjusting device of this type is to be able to precisely and rapidly switch over the load-rigid response of a hydrostatic drive to a load-flexible response of the hydrostatic drive and vice versa, it preferably being possible for the sensitivity of the load-flexible response to be set, particularly preferably in an infinitely variable manner. Here, the adjusting device is to be capable of being realized in a robust way and such that it manages with few components with a low space requirement and low costs. Furthermore, the use of sensor systems and/or electronics is to be restricted to a minimum.